The invention relates to an engine cooling system provided with means for controlling the pressure in different sections of the cooling system during different engine operating modes. This allows one section to be pressurized during a cold start to avoid cavitation, while another circuit can be protected from excessive pressure when the engine is operated at high speed.
Due to a number of factors, such as stricter emission standards and more accessories requiring cooling, the demand for cooling of engine components and accessories is continuously increasing. Consequently, future vehicle engines, in particular truck engines, will require a higher coolant flow compared to current production engines to cope with the increased demand. Increasing the flow of coolant may, however, cause a number of problems.
An increase of the coolant flow through a radiator may result in a larger pressure drop across the radiator than the current design can withstand. The coolant flow may become high enough to cause internal erosion inside the radiator core. An increased coolant flow will normally improve the heat rejection, or cooling capacity, of the radiator, but the coolant flows in current radiators are often so high that the radiators are already saturated on the coolant side. Hence, an additional increase in coolant flow may only give a very slight increase in heat rejection.
Additional problems relating to cooling of vehicle engines involves the risk of cavitation in the engine block and the failure of engine heat exchangers such as EGR-coolers due to the effect of the coolant boiling in local hot-spots. The above problems may at least partially be avoided by increasing the pressure in the engine cooling system. The maximum pressure that can be used in the cooling system is limited by the design pressure of the radiator.
A conventional solution involves using a closed cooling system with an expansion tank and a pressure relief valve. During operation of the engine the coolant is heated up and the engine coolant volume increases to a predetermined level. Pressure variations may be controlled by the expansion tank. If the system becomes overheated the pressure in the cooling system increases up to a maximum allowed pressure and the pressure relief valve is opened for venting excess pressure to the atmosphere.
One problem with an engine cooling system of this type is that the increased system pressure adds to the pressure drop over radiator. The total pressure drop may therefore become too high for the radiator resulting in coolant leaks or even burst coolant conduits or tubes. On the other hand, there is no or a very low pressure in the cooling system during a cold start when the engine temperature is relatively low. Hence, a local build-up of heat in the engine may cause cavitation to occur in coolant conduits in the engine during a cold start before the cooling system pressure builds up.
The problem of lack of pressure in the cooling system during start-up can be solved by pressurizing the system with air from air-brake system. In this way pressurized air can be supplied to the expansion tank, or similar, to achieve a pressure increase at once when the engine is started. However, this solution will not solve the problem relating to a large pressure drop over the radiator.
In order to protect the radiator from excessive pressures, a pressure sensitive by-pass valve can be installed. This will limit the pressure drop over the radiator to an acceptable level and direct at least a part of the coolant flow into a by-pass conduit connected between the valve and a conduit downstream of the radiator. However, the use of this type of valve will require a relatively long time for pressurizing the cooling system during a cold start.
The above problems relating to cavitation in the engine caused during a cold start and coolant flows causing an excessive drop across a radiator are solved by an improved cooling system according to the invention.
According to a preferred embodiment, the invention relates to an engine cooling system comprising a coolant circuit extending through an engine, wherein a coolant flows through the coolant circuit. The engine is preferably a vehicle engine, but the invention can also be used for marine engines or stationary engines. A pump is provided for circulating coolant under pressure through the coolant circuit and a radiator is provided for cooling coolant passing through the coolant circuit. The pump is preferably, but not necessarily, a centrifugal pump. The coolant circuit further comprises a bypass conduit, wherein the by-pass conduit allows coolant to by-pass the radiator and return to the pump. A flow control valve means is arranged for regulating the flow rate of coolant flowing through the radiator and the by-pass conduit and a controller is provided for controlling the flow control valve means in response to input signals from at least one pressure sensor and at least one temperature sensor in the coolant circuit. The controller may be a separate electronic control unit (ECU), connected to at least the said sensors, or a main ECU for controlling the engine operation, connected to these and additional sensors for monitoring all relevant engine related parameters. The flow control valve means may comprise a first controllable valve located in the coolant circuit upstream of the radiator and downstream of the by-pass conduit. A second controllable valve may be located in the bypass conduit.
The first and second individually controllable valves may be analogue valves that can be controlled steplessly between a closed and an open position. An example of valves suitable for this purpose may be electrically or solenoid operated one-way valves. The valves may be arranged to take up any position between fully open and completely closed. Normal operation is preferably, but not necessarily that one valve opens while the other closes.
During a first mode of operation the first and second controllable valves are controlled simultaneously, wherein the total flow through the valves is equal to the flow delivered by the pump. By throttling the valves, the pressure across the pump increases in order to pressurize the system. This mode is in operation after a cold start of the engine, when the pressure in the coolant system is relatively low and the temperature is near the ambient temperature. The first mode of operation is used in order to achieve a relatively rapid pressurization of the section of the coolant circuit that passes through the engine. This mode is typically in operation immediately after a cold start of the engine.
Initially during the cold start mode both the first and second valves will be closed. A limited, controlled leakage through the bypass circuit may be permitted during the initial stage of the pressurization to avoid surge in the pump. The pump is located upstream of the engine and will deliver a relatively high pressure, as there is no or very little flow. A suitable pump for this purpose is preferably, but not necessarily, a centrifugal pump, which is often used in the coolant circuit of truck engines or similar. The coolant will initially be relatively cold and the system pressure in an expansion tank connected to the coolant circuit will be relatively low.
The controller may maintain the first controllable valve in a closed position and controls the second controllable valve in response to the input from a pressure sensor in the coolant circuit downstream of the engine. The second controllable valve may be controlled to maintain a predetermined minimum pressure in the coolant circuit through the engine. Once a desired pressure has been established In the part of the cooling circuit comprising the engine and the by-pass conduit, the controller may control the first controllable valve and/or the second controllable valve in response to the input from a temperature sensor in the coolant circuit downstream of the engine.
The controller may also control the first and second controllable valves in response to the input from a temperature sensor that is preferably, but not necessarily, located in the coolant circuit immediately downstream of the pump. The temperature sensor may alternatively be located in a suitable location between the radiator and the pump. If relatively cold coolant from the initially closed circuit containing the radiator enters parts of the coolant circuit containing the engine block with its cylinder liners, an optional EGR-cooler and similar relatively hot components, then the hot components may experience a thermal shock. If the temperature sensor downstream of the pump senses that the coolant from the radiator is below a predetermined limit, then the flow trough the first valve will be reduced and the flow through the second valve will be increased a corresponding amount. This control of the first valve also prevents relatively hot coolant from the engine from causing a thermal shock in the part of the cooling system containing the relatively cold radiator. The temperature is monitored until the radiator has reached a nominal operating temperature.
In this way components such as cylinder liners, EGR-coolers and similar will by supplied with coolant at a relatively high pressure (system pressure plus pump pressure) immediately after start. This prevents a local build-up of heat from causing cavitation adjacent the cylinder liners in the engine block and other parts of the pressurized coolant conduits of the engine.
During a second mode of operation the first and second controllable valves are controlled simultaneously or substantially simultaneously, wherein the total flow through the valves is equal to or substantially equals the flow delivered by the pump. The second mode of operation is used in order to control the pressure in the section of the coolant circuit that passes through the radiator. During periods where the engine is operated under a high load and/or a high engine speeds it is desirable to increase the cooling capacity of the cooling system. The coolant flow and pressure delivered by a fixed displacement pump driven by the engine is dependent on the engine speed. Hence a relatively high engine speed will result in a relatively high coolant flow and an increased system pressure.
Alternatively an increase in the coolant flow may be achieved by increasing the speed of an electrically driven pump or controlling a variable displacement pump, which increases both the coolant flow and the pressure in the cooling system.
An increased system pressure adds to the pressure drop over the radiator and it is therefore desirable to control the pressure of the coolant entering the radiator inlet. The controller will monitor at least the pressure and temperature of the coolant downstream of the engine and the pressure at the inlet of the radiator. The latter pressure is sensed by a second pressure sensor, located between the first valve and the radiator inlet. When the pressure at the radiator inlet approaches a maximum allowable value the radiator will be near its maximum cooling capacity. At this point the radiator is almost saturated on the coolant side an increase in the coolant flow through the radiator will only have a minor effect on the heat rejection to the atmosphere. As long as the radiator inlet pressure, and hence the total pressure drop over the radiator, is less than or equal to a predetermined maximum value the first controllable valve will be nearly fully open and the second controllable valve will be partially open. However, should the inlet pressure exceed this value, the controller will control the first controllable valve to limit the coolant pressure in the radiator to a predetermined maximum value.